1. Field of the Invention
The present invention is generally directed to the analysis of underground earth formations, and, more particularly, to the determination of formation resistivity properties and/or profiles.
2. Description of Related Art
Electromagnetic (EM) logging tools have been employed in the field of subsurface exploration for many years. These logging tools or instruments entail an elongated support equipped with antennas that are operable as sources or sensors. The antennas on these tools are generally formed as loops or coils of conductive wire. In operation, a transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (“mud”) and into the surrounding formations. The emitted energy interacts with the borehole and formation to produce signals that are detected and measured by one or more receiver antennas. The detected signals reflect the interaction with the mud and the formation. The measurements are also affected by mud filtrate invasion that changes the properties of the rock near the wellbore. By processing the detected signal data, a log or profile of the formation and/or borehole properties is determined.
Conventional logging techniques include “wireline” logging and logging-while-drilling (LWD) or measurement-while-drilling (MWD). A developing method, sometimes referred to as logging-while-tripping (LWT), involves sending a small diameter “run-in” tool through the drill pipe to measure the downhole properties as the drill string is extracted or tripped out of the hole. These logging techniques are well known in the art.
A coil or loop-type antenna carrying a current can be represented as a magnetic dipole having a magnetic moment strength proportional to the product of the current and the area encompassed by the coil. The magnetic moment direction can be represented by a vector perpendicular to the plane of the coil. In the case of more complicated coils, which do not lie in a single plane (e.g. saddle coils as described in published U.S. patent application No. 20010004212 A1, published Jun. 21, 2001), the direction of the dipole moment is given by:ƒr×dland is perpendicular to the effective area of the coil. This integral relates to the standard definition of a magnetic dipole of a circuit. Integration is over the contour that defines the coil, r is the position vector and dl is the differential segment of the contour.
In conventional EM induction and propagation logging tools, the transmitter and receiver antennas are typically mounted with their axes along, or parallel, to the longitudinal axis of the tool. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). An emerging technique in the field of well logging is the use of tools with tilted antennas, i.e., where the antenna's magnetic moment or axis is not parallel to the support axis, or tools with transverse antennas, i.e., where the antenna's magnetic moment or axis is at 90 degrees to the support axis. These tools are thus implemented with antennas having a transverse or tilted magnetic dipole moment/axis (TMD). One logging tool configuration comprises triaxial antennas, involving three coils with magnetic moments that are not co-planar. The aim of these TMD configurations is to provide EM measurements with directed sensitivity. Logging tools equipped with TMDs are described in U.S. Pat. Nos. 6,044,325, 4,319,191, 5,115,198, 5,508,616, 5,757,191, 5,781,436 and 6,147,496.
EM propagation tools measure the resistivity (or conductivity) of the formation by transmitting radio frequency signals into the formation and using receivers to measure the relative amplitude and phase of the detected EM signals. These tools transmit the EM energy at a frequency in the range of about 0.1 to 10 MHz. A propagation tool typically has two or more receivers disposed at different distances from the transmitter(s). The signals reaching the receivers travel different distances and are attenuated to different extents and are phase-shifted to different extents. In analysis, the detected signals are processed to derive a magnitude ratio (attenuation) and phase difference (phase shift). The attenuation and phase shift of the signals are indicative of the conductivity of the formation. U.S. Pat. Nos. 4,899,112 and 4,968,940 describe conventional propagation tools and signal processing.
Resistivity anisotropy is a characteristic of subsurface earth formations that can complicate the evaluation and characterization of potential and existing hydrocarbon-bearing zones. Many reservoir rocks exhibit resistivity anisotropy, especially when saturated with oil. There are several mechanisms, which can produce this anisotropy, among which are very thin sand-shale laminations, depositional changes in clean sandstone, and wind-distributed sands (aeolian formations). Some or all of the individual earth layers can be electrically anisotropic, meaning that the resistivity as measured in one direction along any one layer is different than the resistivity measured in another direction along the layer. Typical anisotropic earth formation layers have a principal resistivity value measured in a direction along the layer's boundaries, generally known as “horizontal resistivity”, and another principal resistivity value measured in a direction perpendicular to the layer's boundaries, generally known as “vertical resistivity”. Collectively, the values of the properties for each layer, the thickness of each layer, and the distances from the wellbore to the boundaries are referred to as “parameters.”
Several prior art tools are available for investigating anisotropic or inhomogeneous formations or formation boundaries. For example, U.S. Pat. No. 5,530,359 discloses a logging tool with multiple transmitter and receiver antennas for detecting locations of formation boundaries. U.S. Pat. No. 6,181,138 discloses a logging tool having skewed antennas for directional resistivity measurements for azimuthal proximity detection of bed boundaries.
There remains a need for improved techniques to indicate and evaluate resistivity anisotropy of potential hydrocarbon-bearing zones in subsurface formations.